In the image forming apparatus such as a laser printer, a laser beam is caused to enter the polygon mirror (rotating polygon mirror) rotating at high-speed of the light deflection apparatus, based on information which has been read out as a measure to write in the image, and thereby, the rotation of the polygon minor forms a scanning beam which gives scanning exposure to a photoreceptor, thus, an image is formed on the photoreceptor.
In the light deflection apparatus wherein the polygon mirror is rotated as a rotor section, high speed rotation of the polygon mirror is carried out, by supporting the rotor section with a dynamic pressure bearing.
In general, a rotation mechanism employing the dynamic pressure bearing is one having therein a thrust bearing, a radial bearing and a rotor section that can rotate against these dynamic pressure bearings, wherein an air gap with a thickness of several μm is formed by air pressure between a pressure surface of the rotor section and a pressure surface of the dynamic pressure bearing which face each other, and by the rotation of the rotor section and by actions of a dynamic pressure generating groove provided on the dynamic pressure bearing, to reduce resistance between the dynamic pressure bearing and the rotor section, thus, the high-speed rotation is possible.
Each of Unexamined Japanese Patent Application Publication No. 8-196056 and Unexamined Japanese Patent Application Publication No. 7-20397 discloses a light deflection apparatus employing a dynamic pressure bearing.
In Unexamined Japanese Patent Application Publication No. 8-196056, dynamic pressure is generated between a radial bearing and a rotor section, while, in the thrust direction, the rotor section having the polygon mirror is caused to float by magnetic suction force by a permanent magnet provided on the radial bearing and by a magnetic body provided on the rotor section. On the end portion in the thrust direction, there is provided an orifice that causes air to circulate.
Even in Unexamined Japanese Patent Application Publication No. 7-20397, dynamic pressure is caused to be generated between the radial bearing and the rotor section, and the rotor section is caused to float by magnetic suction force by a permanent magnet provided on the radial bearing and by a magnetic body provided on the rotor section, in the same way as in Unexamined Japanese Patent Application Publication No. 8-196056. On the end portion in the thrust direction in each of Unexamined Japanese Patent Application Publication No. 8-196056 and Unexamined Japanese Patent Application Publication No. 7-20397, there is provided a ventilation hole for air ventilation on a cover body so that air may be ventilated between the dynamic pressure bearing and the outside.
[Patent Document 1] Unexamined Japanese Patent Application Publication No. 8-196056
[Patent Document 1] Unexamined Japanese Patent Application Publication No. 7-20397
In each of Unexamined Japanese Patent Application Publication No. 8-196056 and Unexamined Japanese Patent Application Publication No. 7-20397, the rotor section is caused to float by magnetic suction force between a permanent magnet provided on the dynamic pressure bearing and a magnetic body provided on the rotor section, so that it may be rotated. Then, it causes an air current generated on the dynamic pressure bearing to be ventilated at an end portion in the thrust direction.
In the rotation mechanism wherein an air is ventilated with the outside at the end portion in the thrust direction of the dynamic pressure bearing, as in Unexamined Japanese Patent Application Publication No. 8-196056 and Unexamined Japanese Patent Application Publication No. 7-20397, there is a problem that the rotation mechanism is easily affected by the outside. The problem to be affected easily from the outside is as follows specifically. That is, when shocks or forces are inflicted on the rotation mechanism having therein dynamic pressure bearing sections, there is sometimes an occasion when a trouble that a rotor section comes off takes place. The rotation mechanism having therein the dynamic pressure bearing, sometimes has an occasion wherein the rotor section takes a lower position against the dynamic pressure bearing, because of the relationship with other parts which were incorporated in the case of subassembly of the rotation mechanism. Under this condition, the rotor section comes off in particular, because gravity acts on the rotor section as a force to separate the rotor section from the dynamic pressure bearing.
An objective of the invention is to solve the problem in the conventional rotation mechanism having therein a dynamic pressure bearing, namely to solve the problem that the conventional rotation mechanism is weak against shocks or forces inflicted from the outside.
The aforesaid objective is attained by the following constructions.